The field of the invention is systems and methods for magnetic resonance imaging (MRI). More particularly, the invention relates to systems and methods for imaging subject with arrhythmia, heart rate variability, and other adverse cardiac conditions.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation field B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (NMR) phenomenon is exploited.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The emitted MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
One common clinical application for MRI is cardiac and cardiovascular imaging. Late Gadolinium Enhancement (LGE) is the current gold standard for evaluation of scar and fibrosis in ischemic and non-ischemic patients. The role of LGE in the assessment of patients with ischemic cardiomyopathy has been extensively studied. LGE imaging has also been used for imaging of scar in left atrium and pulmonary veins in patients with atrial fibrillation (AF) (the most common cardiac arrhythmia prevalent in the elderly), in patients with hypertension, and in patients with coronary artery disease (CAD). Two-dimensional (2D) imaging studies are typically used clinically for LGE imaging. Three dimensional (3D) imaging has been introduced as an alternative approach for assessment of scar, which provides better volumetric coverage and higher signal-to-noise-ratio (SNR) than 2D imaging. However, 3D acquisitions take longer to complete and, thus, are more taxing on the patient. While 2D images can be acquired within a short breathhold of 10-15 seconds, 3D images are often 5-10 minutes in duration and, thus, require free-breathing.
In practice, LGE images are acquired after administration of a gadolinium contrast agent using a T1-weighted inversion recovery pulse sequence. The inversion time is selected prior to performing the LGE sequence using a Look-Locker imaging sequence such that the healthy myocardium signal is nulled. Abnormal washout kinetics in infarcted tissue enables the depiction of scar as bright regions against a dark myocardial background. Assuming a constant inversion time throughout the entire scan, the time between two inversion pulses in LGE imaging is preferably selected to equal the duration of the previous heartbeat, such that the heart is in a common portion of the cardiac phase during each successive acquisition. Failure to time the acquisition properly will result in substantial artifacts in the resulting images and will degrade the clinical utility of the images. Unfortunately, while this timing is necessary to avoid serious motion artifacts this time (typically 750-1000 ms) is too short to allow full recovery of the longitudinal magnetization of the cardiac tissue and most of the surrounding structures. Hence, the LGE signal is directly influenced by the length of the previous R-R interval.
This process is further complicated if the heart rate is inconsistent or variable and/or an arrhythmia is present. Such variability will cause a time-varying weighting of k-space lines, which results in ghosting artifacts. Therefore, arrhythmia and heart rate variability are among major factors that could deteriorate image quality, particularly, when performing a 3D LGE imaging study, which requires relatively long scan times.
While LGE is routinely utilized for the detection of large, localized scar areas, its use in detecting diffuse fibrosis is limited, even in sinus rhythm. Myocardial T1 mapping recently emerged as a supplementary sequence in the detection of scar and fibrosis. Due to its quantitative nature, T1 mapping allows for inter- and intra-patient reproducibility, and may facilitate the diagnosis of diffuse fibrosis in the myocardium. However, due to cardiac and respiratory motion, myocardial T1 mapping remains a challenging task.
Recently, the modified Look-Locker inversion recovery (MOLLI) sequence was proposed in Messroghli D R, Radjenovic A, Kozerke S, Higgins D M, Sivananthan M U, Ridgway J P, Modified Look-Locker inversion recovery (MOLLI) for high-resolution T1 mapping of the heart. Magnetic Resonance in Medicine 2004;52(1):141-146, for cardiac T1 mapping, and has been used in clinical and pre-clinical trials. By performing three-to-five data readouts after each preparation pulse, MOLLI incorporates the efficient sampling of the spin-lattice relaxation curve that was originally proposed in Look D C, Locker D R. Time saving in measurement of NMR and EPR relaxation times, Review of Scientific Instruments 1970;41(2):250-251. In order to provide a sufficient number of sampling points of the curve, three groups of images are acquired, each following a single inversion pulse. The three groups contain 3, 3, and 5 ECG-triggered images, respectively, which are acquired in consecutive heartbeats. Two rest periods of multiple heart cycles each separate these three groups in order to allow for sufficient recovery of the longitudinal magnetization. These rest cycles decrease the imaging efficiency and require relatively long breath holds, for example, 17 heartbeats. Furthermore, a fixed set of sample points on the T1 relaxation curve is predetermined by the ECG triggering, which impacts the T1 calculation, resulting in poor fit conditions for short T1 times. Furthermore, the signal disturbance of the relaxation curve induced by imaging varies based on the heart rate. For long T1 times, this results in a pronounced heart rate dependence of the calculated T1 values.
Thus, it would be desirable to have a system and method for an improved magnetization preparation technique to enable assessment of scar and diffuse fibrosis in the presence of arrhythmia or heart rate variability.